A photographically useful reagent exerts different effects when previously incorporated in a photographic light-sensitive material from when incorporated in a processing solution. For example, a photographic reagent which is subject to decomposition in an acidic or alkaline or oxidizing or reducing atmosphere and cannot withstand a prolonged storage in a processing solution can be effectively used while the processing solution composition can be simplified and easily prepared. Furthermore, a necessary photographic reagent can exert its effects only on necessary sites, i.e., certain layers or their adjacent layers in the multi-layer light-sensitive material. Moreover, the content of a photographic reagent can be altered as a function of development of silver halide.
However, when incorporated in an active form, a photographic reagent reacts with other components in the photographic light-sensitive material during storage before development or decomposes under the effect of heat or oxygen and thus cannot often exert expected effects during development. An approach for overcoming such a difficulty is to incorporate such a photographic reagent in the photographic light-sensitive material in the form of substantially inactivated form attained by blocking the active group therein, i.e., in the form of photographic reagent precursor. If such a useful photographic reagent is a fog inhibitor or development inhibitor, the active group can be blocked to inhibit the adsorption thereof by a light-sensitive silver halide in storage or the desensitizing effect by the formation of silver salts as well as to release such a reagent at a required time. This results in an advantage that fogging can be reduced without impairing sensitivity, excess development fogging can be inhibited, or development can be suspended at a required time. If a useful photographic reagent is a developing agent, auxiliary developing agent or fogging agent, an active group or adsorption group contained therein can be blocked to inhibit various photographically dverse effects due to the production of semiquinone or oxide by air oxidation during storage or inhibit the production of fogged nucleus during storage due to the inhibition of electron injection into silver halide. This results in an advantage that a stable processing can be realized. If a photographic reagent is a bleach accelerator or blix accelerator, an active group contained therein can be blocked to inhibit the reaction thereof with other components contained therein. The blocking group can be removed during development to cause required properties to be accomplished at a necessary time.
As mentioned above, the use of a photographic reagent precursor is extremely effective for the full accomplishment of the properties of a photographic reagent. However, such a photographic reagent precursor must meet extremely strict requirements. In other words, the precursor must meet conflicting requirements. In particular, the precursor must occur stably in a photographic light-sensitive material during storage. On the other hand, when the photographic light-sensitive material is processed, the blocking group is removed at a necessary time to cause the photographic reagent to be readily and effectively released.
Some approaches for blocking photographic reagents have already been known. For example, blocking groups such as acyl group and sulfonyl group can be used as disclosed in JP-B-48-9968 and JP-B-47-44805 (U.S. Pat. No. 3,615,617) (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-52-8828 and JP-A-57-82834 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and U.S. Pat. No. 3,311,476. A blocking group which undergoes a so-called reverse Michael reaction to release a photographically useful reagent can be used as disclosed in JP-B-55-17369 (U.S. Pat. No. 3,888,677), JP-B-55-9696 (U.S. Pat. No. 3,791,830), and JP-B-55-34927 (U.S. Pat. No. 4,009,029), and JP-A-56-77842 (U.S. Pat. No. 4,307,175), JP-A-59-105642, and JP-A-59-105640. A blocking group which undergoes an intramolecular electron migration to release a photographically useful reagent with the production of quinonemethide or quinonemethide compounds can be used as disclosed in JP-B-54-39727, U.S. Pat. Nos. 3,674,478, 3,932,480, and 3,993,661, and JP-A-57-135944, JP-A-57-135945, and JP-A-57-136640. An intramolecular ring closure reaction can be utilized as disclosed in JP-A-55-53330 and JP-A-59-218439. The cleavage of a 5- or 6-membered ring can be used as disclosed in JP-A-57-76541 (U.S. Pat. No. 4,335,200), JP-A-57-135949, JP-A-57-179842, JP-A-59-137945, JP-A-59-140445, JP-A-59-219741, and JP-A-60-41034.
These known approaches are disadvantageous in that a photographic reagent precursor stable in storage releases a photographic reagent too slowly, requiring the processing with a high alkali as a pH value of 12 or more or in that even if a photographic reagent precursor can release a photographic reagent at a sufficient rate in a processing solution with a pH value of 9 to 12, it decomposes gradually during storage, loosing the effects of the precursor.
These disadvantages can be attributed to the fact that the release of a photographically useful reagent from the blocked photographic reagent must rely on the attack by OH.sup.- ions. In other words, if the conventional photographic light-sensitive material is developed at a pH value of 9 to 12, the difference in OH.sup.- ion concentration between during storage and processing of the photographic light-sensitive material is 10.sup.2 to 10.sup.5 because the pH value reaches 6 to 7 during storage. Accordingly, it is presumed that a blocked photographic reagent which releases a photographically useful group at a half-life period of 3 minutes (3 minutes are required for half the added amount of the photographic reagent to decompose) at a pH value of 10 for example decomposes at a half period of 3 minutes.times.10.sup.4 (=30,000 minutes=about 500 hours) during storage (pH=6). This means that if a photographic reagent precursor is stored for about 3 weeks, about half the added amount thereof decomposes, making it impossible to put this system into practical use. A blocked compound which releases a photographic reagent at a half period of 3 minutes during processing at a pH value of 11 decomposes during storage at a half period of about 10 times (about 30 weeks) the case where the processing is effected at a pH value of 10. However, even this half period leaves much to be desired. Thus, these approaches cannot be put into practical use in view of preservability.